If you are involved with electronics and/or electricity, you will find that despite the many complex circuits and equations that exist, you will frequently use the few most common equations and circuit configurations. For example, V = IR is arguably the most important equation in electronics, followed by Kirchhoff’s current and voltage laws.
But there will occasionally be situations where you find yourself needing an uncommon component (such as a gas discharge tube) or equation (e.g., the relationship between inductance and the physical characteristics of a coiled wire). Maybe you, like many others, just need a conversion chart for a resistor value or wire colour codes for power distribution cables.
Here you will find many references for situations such as these, ranging from DC circuit equations and unit conversions to derivatives and troubleshooting techniques.